Titanium oxide is used in a variety of applications, for example, pigments, UV screeners, catalysts, photocatalysts, catalyst supports, adsorbents, ion-exchangers, fillers, reinforcements, ceramic-forming raw materials, precursors to complex oxides such as perovskite complex oxide, and magnetic tape primers.
Among others, photocatalytic titanium oxide fine particles are often used in such applications as substrate surface cleaning, deodorizing and antifungal treatment because a photocatalytic coating film which is formed by coating a dispersion of titanium oxide fine particles onto the surface of various substrates serves to decompose organic matters and renders the film surface hydrophilic due to the photocatalytic action of titanium oxide. However, the contact area of photocatalytic particles with a substance to be decomposed must be increased before the photocatalytic activity can be enhanced. To this end, the particles are required to have a primary particle size of up to 50 nm. Also the film is required to be transparent to avoid any impact on the substrate design.
Dispersions of titanium oxide fine particles are generally prepared, for example, by a method of synthesizing titanium oxide fine particles by the vapor phase or liquid phase process and dispersing the particles in a dispersing medium with a dispersant such as an organic polymeric dispersant on a wet dispersing machine (see JP-A H01-003020, JP-A H06-279725), JP-A H07-247119). However, these methods have the problem that since ultrafine particles with an average particle size of up to 50 nm are prone to agglomerate together, a great amount of labor is necessary until dispersion to primary particles, and in some cases, dispersion to primary particles is impossible.
It is also disclosed that an anatase titanium oxide dispersion having long-term stability is prepared by dissolving titanium hydroxide with hydrogen peroxide to form a peroxotitanic acid solution and subjecting it to hydrothermal treatment (JP-A H10-67516); and that a rutile titanium oxide dispersion is prepared by hydrothermal treatment of peroxotitanic acid solution in the co-presence of a tin compound (JP-A H01-78928). These methods have the problem that hydrothermal treatment requires a relatively long reaction time of 40 to 2 hours at 85 to 200° C., for example. Besides, a high efficiency preparation method has not been established because of batchwise reaction.